1. Field of the Invention
The present invention relates to an actuator configured so that output rotation of a motor is reduced in speed and output via a wave gear drive. The present invention more specifically relates to a method for compensating for angular transmission error of an actuator, the method being capable of eliminating or reducing an angular transmission error component (a non-linear elastic deformation component) resulting from elastic deformation of a flexible externally-toothed gear of the wave gear drive.
2. Description of the Related Art
(Current State of Angular Transmission Error Compensation for an Actuator Provided with a Wave Gear Drive)
Angular transmission error degrades the positioning response of an actuator provided with a wave gear drive. The components of this angular transmission error in the prior art are a motor-shaft synchronized component (fixed component), which is synchronized to the rotation of a motor shaft (input shaft); an FS-WG relative-rotation synchronized component (mobile component) arising from relative rotation between a flexible externally-toothed gear (FS—[flex spline]) and a wave generator (WG); and a load-shaft synchronized component, which is synchronized to the rotation of the actuator output shaft. Among these components, compensation for the motor-shaft synchronized component has been actively performed.
In JP-A 2002-175120, angular transmission error data of an actuator output shaft is measured relative to the rotational positions of one revolution of a motor rotating shaft for an actuator provided with a wave gear drive. Positional feedback control is performed on the actuator output shaft on the basis of these measurements. Feedback control for positioning an actuator output shaft is similarly proposed in JP-A 2002-244740. In JP-A 2006-039958, a dividing system for performing an index-table dividing operation using an actuator provided with a wave gear drive is proposed. In this dividing system, driving control for an actuator and the index-table dividing operation are performed according to a set dividing-operation pattern. The stored operation pattern can circumvent or minimize oscillations occurring during acceleration and deceleration of the actuator as a result of the elasticity of the flexible externally-toothed gear, and positioning can be performed in a short period of time.
The angular transmission error of an actuator provided with a wave gear drive includes a non-linear elastic deformation component that is not synchronized to rotation and that occurs due to elastic deformation of the flexible externally-toothed gear of the wave gear drive. The proportions of the motor-shaft synchronized component (fixed component) and the non-linear elastic deformation component in the angular transmission error are large when overshoot occurs in an actuator provided with a wave gear drive, as shown in Table 1. The non-linear elastic deformation component must therefore be removed or reduced in order to improve the positioning precision of the actuator output shaft as shown in the column “Goal” in Table 1.
TABLE 1Proportions of angular transmission error during incidence ofovershootNoCurrentcompensationstateGoalCompensationNon-linear elastic43%43%0%possibledeformationcomponentMotor-shaft41%5%5%synchronizedcomponent (fixedcomponent)CompensationFS-WG relative-11%11%11%not possiblerotationsynchronizedcomponent (mobilecomponent)Load-shaft5%5%5%synchronizedcomponentTotal100%64%21%
In the prior art, an amount of correction corresponding to the component synchronized to the motor shaft was added to the positioning command or positioning feedback as proposed in JP-A 2002-175120 and JP-A 2002-244740, whereby the motor-shaft synchronized component (fixed component) was reduced or eliminated, and positioning precision was improved. However, an effective method was not proposed for compensating for the non-linear elastic deformation component that is not synchronized to rotation and that occurs due to elastic deformation of the flexible externally-toothed gear. The only such method is the method for performing operational control proposed in JP-A 2006-039958, in which the elasticity of the flexible externally-toothed gear is considered beforehand in the standard operational pattern.
Measurement results for the FS-WG relative-rotation synchronized component (mobile component) have poor reproducibility. The amplitude of the load-shaft synchronized component changes depending on the assembly state of the load, and means for measuring the absolute angle of the load shaft do not exist. Therefore, compensation cannot be performed for the FS-WG relative-rotation synchronized component (mobile component) and the load-shaft synchronized component.
Compensation for the non-linear elastic deformation component along with the motor-shaft synchronized component must therefore be performed in order to increase the positioning precision of an actuator provided with a wave gear drive.